Today the communications between the ground and moving objects above the earth's atmosphere (satellites and space objects in general) are made almost exclusively by radio frequency signals and because of the increasing demand for channels and bandwidth, higher and higher frequencies are used.
Due to the congestion of radio frequency bands currently used for communication with satellites, bands L, S, C and Ku, and with X-band reserved for military applications, it has become very difficult to obtain new allocations of channels in the frequency range of 1 GHz to 18 GHz as covered by these bands.
Link systems with satellites are therefore moving towards the use of even higher frequencies of 18 GHz to 40 GHz, referred to as bands K and Ka.
These higher frequencies implemented for these communications will produce real benefits.
Use of higher frequencies allows for higher bandwidths and thus facilitates dedicated broadband services that require a lot of data such as telephony, video conferencing, digital television, and especially high-speed satellite internet.
K and Ka bands are currently under-used, which facilitates the allocations of channels.
High frequencies can also allow the use of smaller antennas for a given antenna “gain”, which is advantageous in many circumstances, especially for mobile installations or discreet military installations, and also for fixed installations in reducing the size of the antenna infrastructure.
This reduction in size of the antennas is also an advantage in the case of the tracking of space objects or during launch phases.
During the critical phases of satellite launch, orbit placement or orbit parking, a guarantee of quality of the link is required, which leads to the use of tracking antennas of large diameters. However, the control of the antenna tracking is complex not only because of the antenna size but also because of their high directivity.
Higher frequencies for communications is not without its difficulties because the high-frequency signals are in general (and especially the frequency bands K, Ka) sensitive to atmospheric phenomena.
In practice the propagation conditions deteriorate depending on local atmospheric characteristics where the receiving antenna is located. The signals are weakened especially by humidity and the presence of clouds or rain.
In the worst cases the signals from a satellite cannot be intelligibly received and the communication link is no longer available.
To overcome these difficulties and to maintain communication links with satellites and other space objects, several solutions are currently implemented.
One solution is to maintain communication by increasing the transmitter power when the conditions deteriorate to counter the attenuation introduced by the poor propagation conditions. Relatively simple to implement as long as this feature was built-in at the transmitter at the design stage, however, this solution causes interference and rapidly increases rates of intermodulation levels, which limits the possibilities of its use.
Another solution is to adapt the transmitted signal and reduce the bandwidth in order to match the available link budget. This solution is relatively complex to implement, lacks responsiveness and above all implies a lower transfer rate that may be incompatible with the expected service.
Other solutions are based on a technique of re-routing the transmission by another ground station or by a relay satellite, which can overcome local constraints where the transmission conditions between the ground and the satellite are unfavorable.
These solutions are very effective as they allow communications at the initial performance levels, but they are complex and imply a high cost due to the back-up ground stations.
These various solutions can if necessary be combined to improve the performance, or alternatively the specific components of a receiver chain can be improved, especially the size of the antenna or the sensitivity and signal to noise ratio of the receivers.
It is in this context that the option to cool the amplifiers is known to improve the signal to noise ratio, by reducing the internal noise of the amplifiers.
The components of the receiver (RF amplifiers, filters, etc.), in this case, are associated with a cryogenic cooler in an insulating enclosure such as a Dewar (named after the inventor) or cryostat to cool the components to a very low temperature of usually approximately 100 Kelvin or lower.
In typical system designs, components including the antennas are designed to meet the nominal performance in the worst conditions that can be encountered, during which the communication link must be maintained.
Atmospheric conditions leading to deterioration of propagation are generally localized and temporary, with the result that the systems are designed with significantly higher performance ratings than is required for a large proportion of their operational life.
The resulting increase in the complexity of the components and their maintenance, results in a significant increase in their build cost, operational costs and in a lower reliability.